Polyurethane elastic yarn obtained from 4,4'-diphenylmethane diisocyanate, a polyhydroxy polymer having a relatively low degree of polymerization and a compound containing polyfunctional active hydrogen, has a high degree of rubber elasticity, is excellent in mechanical properties such as tensile stress and resilience and has an excellent thermal behavior. Therefore, much attention has been called to this yarn as a functional fiber material for foundation garments, socks, sportswear, and the like.
However, it is known that, when such products composed mainly of a long chain segmented polyurethane are washed with chlorine bleach, a marked deterioration in the physical properties of the segmented polyurethane takes place.
Moreover, it is known that, when swimsuits composed of polyurethane elastic yarn and polyamide yarn are exposed to chlorinated water in a swimming pool having an active chlorine concentration of 0.5 to 3 ppm, deterioration of the physical properties of the polyurethane elastic yarn and discoloration and fading of dyestuff adhering to the polyamide yarn occur.
Accordingly, for the purpose of improving chlorine resistance of the polyurethan elastic yarn used for swimming race suits worn in swimming pools, a polyurethane prepared by the use of a polyester, as a polyhydroxy polymer having a low degree of polymerization, which is made more excellent in chlorine resistance due to the molecule bonding viewpoint thereof, has been mainly used. However, an aliphatic polyester has high biological activity, and therefore, the polyester based polyurethane has a disadvantage that it is likely to be attacked by fungi. In addition, the chlorine resistance of the polyester based polyurethane has not been satisfactory. Moreover, treatments of the polyester based polyurethanes with a tannin solution after dyeing treatments are currently widely used as a method for preventing swimsuits from becoming discolored an faded by chlorine.
Various additives have heretofore been proposed to improve the resistance of the polyurethane elastic yarn to chlorine-induced deterioration. For example, Japanese Unexamined Patent Publication (Kokai) No. 57-29609 discloses zinc oxide as an additive.
However, zinc oxide has a disadvantage that it is removed from the yarn by dyeing treatment thereof in an acidic condition (pH 3-4), and the remaining amount of zinc oxide therein is markedly decreased, whereby the chlorine resistance is also greatly lowered.
The present applicant has previously proposed a polyurethane composition in which the chlorine resistance is improved by the use of hydrotalcite, as a polyurethane composition with improvements over the drawbacks as mentioned above (see Japanese Unexamined Patent Publication No. 59-133248). A less amount of hydrotalcite dispersed in polyurethane elastic yarns is removed therefrom even in dyeing treatment in an acidic condition (pH 3-4), and thus excellent chlorine-resistant properties are realized.
However, hydrotalcite is very likely to agglomerate in polar solvents such as dimethylacetamide and dimethylformamide which are solvents for polyurethane used during spinning. As a result, a rise in discharge pressure and yarn breakage are observed in the process of spinning, and it has been found that stabilized spinning of the yarn over a long period of time is difficult. Furthermore, it has been found that phenomena as described below take place when yarn obtained from the polyurethane composition is used for swimming race suits. That is, the yarn is turned brown in the process of tannin solution treatment for preventing dyestuff for polyamide from being discolored by chlorine; and the yarn swells when immersed in chlorinated water.